Linting with pre-commit

hearding all your linters

Why linting

• adopt proven coding-practices
• code needs to maintained
• prevent bugs
• ease migrations
• simplify code-reviews
  • avoids style debates on future changes

Ansible linting challenges

• Playbooks and roles uses YAML
• YAML linters do not check Ansible syntax
• Ansible syntax check does not cover style or formatting
• Python modules
• Shell scripts
• Generic text errors

pre-commit

the tool, not the git hook

• a meta-linter, or orchestrator
• pre-commit tool does not require a git-hook
• git-hook is totally optional

Terminology

• a pre-commit hook is mainly a linter
• a repo is the source of a linter
• a repo can have multiple hooks defined inside

Installing

pip install pre-commit

Configuring

# .pre-commit-config.yaml
repos:
-   repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v2.3.0
    hooks:
    -   id: end-of-file-fixer
    -   id: trailing-whitespace
-   repo: https://github.com/ansible/ansible-lint
    rev: v4.3.0a0
    hooks:
    -   id: ansible-lint

Running

$ pre-commit run
[INFO] Installing environment for local.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
[INFO] Installing environment for https://github.com/ansible/ansible-lint.git.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...

Fix End of Files.........................................................Passed
Trim Trailing Whitespace.................................................Passed
Mixed line ending........................................................Passed
Check for byte-order marker..............................................Passed
Check that executables have shebangs.....................................Passed
Check for merge conflicts................................................Passed
Check for broken symlinks................................................Passed
Debug Statements (Python)................................................Passed
Flake8...................................................................Passed
Check Yaml...............................................................Passed
yamllint.................................................................Passed
Ansible-lint.............................................................Passed
bashate..................................................................Passed

pre-commit benefits

• lower disk footprint
  • any tool:version is installed only once
• ease bumping
• scales well with lots of linters
• keeps console output clean
• predictable results
  • via enforced use of git tags/revisions

Why not just using old tox

• tox itself uses pre-commit for linting its code
• tox is slower
• tox takes more disk space
• tox can install only python tools

pre-commit limitations

• relies on git cloning
  • Caching ~/.cache/pre-commit on CI/CD highly recommended
• Partial support for containers
  • Not working with remote docker hosts
  • Not working with podman
• Single core developer behind

Most useful pre-commit hooks

• pre-commit-hooks a collection of generic mini-hooks maintained by pre-commit author
• ansible-lint
• yamllint
• flake8, black and mypy for python code
• doc8 for RST docs
• bashate for shell scripts
• you can write your own local check!

I’m sold, how do I start?

• .pre-commit-config.yaml from protogen project
• activate only one repo at a time
• start with more pressing ones
• temporary add rule skips when adding new linter
• gradually remove skips in follow-ups

🤗

That’s it! Any Questions?

• pre-commit.com official documentation
• Follow asottile on twitter or twitch
• Discover existing pre-commit-hooks
• Check exiting tickets

Presentation source is available at sbarnea/slides